This is a "map" visualization, meaning that the images are plotted in a coordinate space — here, a 2D t-SNE embedding of a 45901-dimensional greyscale vector space. The vector space is composed of the full greyscale pixel definitions of anatomical images of brains in an ADHD study at Pitt (admittedly, we're not yet getting great visual sorting here). The gridding principle is "stack", meaning that images are plotted in their exact (binned) locations, and some images may share a bin. Visually, then, they simply stack atop each other and only one is visible. However, the number of bins (and so the amount of stacking) is subject to a duplication criterion of < 10%, so at most, there is 10% stacking. Each image is annotated with two meters, one above the brain and one below, each with a bar whose color codes for a categorical variable and whose length codes for a continuous variable. The color of the top bar (the bar above the brain) codes for handedness (red = RH), the bottom gender (unknown, only "0" and "1" in the data). The length of the top bar is IQ percentile, the bottom age percentile.